The present disclosure relates to nitride semiconductor light emitting chips, and nitride semiconductor light emitting devices including the same.
Nitride semiconductors containing nitrogen (N) as a group V element have been expected as a material of a short wavelength light emitting element because of their band gap size. Gallium nitride-based compound semiconductors, in particular, have been actively researched, and blue light emitting diodes (LEDs), green LEDs, and blue semiconductor laser diodes that use a gallium nitride-based compound semiconductor have been also commercialized.
Gallium nitride-based compound semiconductors include a compound semiconductor obtained by substituting at least one of aluminum (Al) or indium (In) for part of gallium (Ga). Such a nitride semiconductor is represented by the general formula AlxGayInzN (where 0≦x, z<1, 0<y≦1, and x+y+z=1). The gallium nitride-based compound semiconductors are hereinafter referred to as GaN-based semiconductors.
The replacement of Ga atoms with Al atoms in a GaN-based semiconductor allows the band gap of the GaN-based semiconductor to be wider than that of GaN, and the replacement of Ga atoms with In atoms in a GaN-based semiconductor allows the band gap of the GaN-based semiconductor to be narrower than that of GaN. Thus, not only short wavelength light, such as blue or green light, but also long wavelength light, such as orange or red light, can be emitted. From such a feature, nitride semiconductor light emitting elements have been expected to be used for, e.g., image display devices and lighting devices.
Nitride semiconductors have a wurtzite crystal structure. In FIGS. 1A, 1B, and 1C, the plane orientations of the wurtzite crystal structure are expressed in four-index notation (hexagonal indices). In four-index notation, crystal planes and the orientations of the planes are expressed using primitive vectors expressed as a1, a2, a3, and c. The primitive vector c extends in a [0001] direction, and an axis in this direction is referred to as a “c-axis.” A plane perpendicular to the c-axis is referred to as a “c-plane” or a “(0001) plane.” FIG. 1A illustrates, not only the c-plane, but also an a-plane (=(11-20) plane) and an m-plane (=(1-100) plane). FIG. 1B illustrates an r-plane (=(1-102) plane), and FIG. 1C illustrates a (11-22) plane. Herein, the symbol “-” attached to the left of one of parenthesized numbers indicating the Miller indices expediently indicates inversion of the number, and corresponds to each of “bars” in some of the drawings.
FIG. 2A illustrates a crystal structure of a GaN-based semiconductor using a ball-and-stick model. FIG. 2B is a ball-and-stick model obtained by observing atomic arrangement in the vicinity of the m-plane surface from an a-axis direction. The m-plane is perpendicular to the plane of the paper of FIG. 2B. FIG. 2C is a ball-and-stick model obtained by observing atomic arrangement of a +c-plane surface from an m-axis direction. The c-plane is perpendicular to the plane of the paper of FIG. 2C. As seen from FIGS. 2A and 2B, N atoms and Ga atoms are located on a plane parallel to the m-plane. On the other hand, as seen from FIGS. 2A and 2C, a layer in which only Ga atoms are located, and a layer in which only N atoms are located are formed on the c-plane.
Conventionally, when a semiconductor element is to be fabricated using a GaN-based semiconductor, a c-plane substrate, i.e., a substrate having a (0001) plane as its principal surface, has been used as a substrate on which a nitride semiconductor crystal is grown. In this case, spontaneous electrical polarization is induced in the nitride semiconductor along the c-axis due to the arrangements of Ga and N atoms. Thus, the “c-plane” is referred to as a “polar plane.” As a result of the electrical polarization, a piezoelectric field is generated in a quantum well layer forming a portion of a light emitting layer of a nitride semiconductor light emitting element and made of InGaN along the c-axis. Due to the generated piezoelectric field, the distributed electrons and holes in the light emitting layer are displaced, and the internal quantum efficiency of the light emitting layer is decreased due to a quantum-confined Stark effect of carriers. In order to reduce the decrease in the internal quantum efficiency of the light emitting layer, the light emitting layer formed on the (0001) plane is designed to have a thickness equal to or less than 3 nm.
Furthermore, in recent years, consideration has been made to fabricate a light emitting element using a substrate having an m- or a-plane called a nonpolar plane, or a -r- or (11-22) plane called a semipolar plane as its principal surface. As illustrated in FIG. 1A, m-planes of the wurtzite crystal structure are parallel to the c-axis, and are six equivalent planes orthogonal to the c-plane. For example, in FIG. 1A, a (1-100) plane perpendicular to a [1-100] direction corresponds to one of the m-planes. The other m-planes equivalent to the (1-100) plane include a (-1010) plane, a (10-10) plane, a (-1100) plane, a (01-10) plane, and a (0-110) plane.
As illustrated in FIGS. 2A and 2B, Ga and N atoms on the m-planes are present on the same atomic plane, and thus, electrical polarization is not induced in directions perpendicular to the m-planes. Therefore, when a light emitting element is fabricated using a semiconductor layered structure having an m-plane as its growth surface, a piezoelectric field is not generated in a light emitting layer, and the problem where the internal quantum efficiency is decreased due to the quantum-confined Stark effect of carriers can be solved. This applies also to the a-plane that is a nonpolar plane except the m-planes, and furthermore, even when, instead of the m-plane, the -r- or (11-22) plane called the semipolar plane is used as the growth surface, similar advantages can be provided.